Abstract
Between 1992 and 1998, we sampled gas discharges from ≤173°C fumaroles and springs
at 12 quiescent but potentially restless volcanoes in the Cascade Range and Aleutian Arc
(CRAA) including Mount Shasta, Mount Hood, Mount St. Helens, Mount Rainier, Mount Baker,
Augustine Volcano, Mount Griggs, Trident, Mount Mageik, Aniakchak Crater, Akutan, and
Makushin. For each site, we collected and analyzed samples to characterize the chemical (H2O,
CO2, H2S, N2, CH4, H2, HCl, HF, NH3, Ar, O2, He) and isotopic (δ13C of CO2, 3He/4He, 40Ar/36Ar,
δ34S, δ13C of CH4, δ15N, and δD and δ18O of water) compositions of the gas discharges, and to
create baseline data for comparison during future unrest. The chemical and isotopic data show
that these gases contain a magmatic component that is heavily modified from scrubbing by deep
hydrothermal (150° - 350°C) water (primary scrubbing) and shallow meteoric water (secondary
scrubbing). The impact of scrubbing is most pronounced in gas discharges from bubbling
springs; gases from boiling-point fumaroles and superheated vents show progressively less
impact from scrubbing. The most effective strategies for detecting gas precursors to future
CRAA eruptions are to measure periodically the emission rates of CO2 and SO2, which have low
and high respective solubilities in water, and to monitor continuously CO2 concentrations in soils
around volcanic vents. Timely resampling of fumaroles can augment the geochemical
surveillance program by watching for chemical changes associated with drying of fumarolic
pathways (all CRAA sites), increases in gas geothermometry temperatures (Mount Mageik,
Trident, Mount Baker, Mount Shasta), changes in δ13C of CO2 affiliated with magma movement
(all CRAA site), and increases in 3He/4He coupled with intrusion of new magma (Mount Rainier,
Augustine Volcano, Makushin, Mount Shasta). Repose magmatic degassing may discharge
substantial amounts of S and Cl into the edifices of Mount Baker and several other CRAA
volcanoes that is trapped by primary and secondary scrubbing. The consequent acidic fluids
produce ongoing alteration in the 0.2- to 3-km-deep hydrothermal systems and in fields of
boiling-point fumaroles near the surface. Such alteration may influence edifice stability and
contribute to the formation of more-hazardous cohesive debris flows. In particular, we
recommend further investigation of the volume, extent, and hazards of hydrothermal alteration at
Mount Baker. Other potential hazards associated with the CRAA volcano hydrothermal systems
include hydrothermal eruptions and, for deeper systems intruded by magma, deep-seated edifice
collapse.
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First posted January 23, 2004
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